Reactive Armor, Smoke, APSFD whatever they are called shells if you have them, if not go with the TOW I guess.

One team mate to gun for you with a repair torch. Preferable if he can equip proximity scan as well. If your goal is only finish this assignment, hang back and be non aggressive until you know where the enemy IFV is, then try to sneak up on it. Upon engagement with IFV, have gunner jump out, move to the back and start repairing damage as soon as you see it. Also try to keep an eye out for enemy IFV employing this same tactic. If properly ambushed from behind this really shouldn't be an issue but ya never know.

Might get your repair person to try and land an SMAW on it at the start of the engagement, just to get that extra bit of damage, tip of the spear and all that junk. Don't let him shoot after the vehicle is disabled as he could steal your kill and render the exercise moot.

Edit: Okay so you've really not got much to work with on an IFV do you? Scratch that above. if you can, go with smoke, TOW, and auto loader instead. It'll let you fire more rapidly. If you're going against people who have the preferred setup I listed above, I can see why you are not having much luck there. Absolutely go for an ambush from behind, let your engineer shoot first to knock off the reactive if they have it. Then hit with standard rounds, TOW, and back and forth till they are dead. If you have the element of surprise this should work as long as they don't get a repair team working on them too.

If that doesn't work with you, you could try to get crafty, let someone else you know with the APFSD shells drive and disable the IFV, then switch seats so you can finish it off. I'd only recommend this if you are getting really desperate though. Might just want to practice some IFV driving, rank that thing up some then give it another go.